The invention concerns a connector of the nut-and-bolt type with a bolt that has an external thread and/or a nut that has an internal thread and with a locking device that provides a moment of friction that counteracts the unscrewing moment. The invention can be realized in bolts manufactured for use with nuts of standard pitch for example. The nut can be in the form of a threaded bore in a solid structure, the engine of a motor vehicle for example. It is on the other hand also possible for the invention to be embodied in a nut intended for use with a bolt of standard pitch. Finally, the invention can be embodied in either a bolt or in a nut if both have non-standard pitches that match.
The failure of a connector of the nut-and-bolt type is as is known ascribed either to loosening or to unscrewing. A nut-and-bolt type connector becomes loose when the binding power decreases or vanishes subject to axial stress due either to inadequate initial tightening, too high a setting, or excessive relief of the attached components, etc. What makes a nut-and-bolt type connector unscrew on the other hand are usually oscillating, jolting, or vibrating transverse forces perpendicular to the axis of the bolt. The friction on the threads and/or below the head of the bolt and hence the persistence of the connection accordingly soon decrease or vanish completely. The present invention aims at a locking device that will prevent unintended unscrewing.
A connector of the aforesaid type already has a locking device. The moment of friction that counteracts the unscrewing moment is provided by applying a microencapsulated adhesive to either the external or internal thread. When the nut and bolt are screwed together, the adhesive is activated and cures in the absence of air. This increases the moment of friction in the thread. The microencapsulated adhesive can be adapted to various conditions and will also resist mechanical stress and temperatures up to approximately 110.degree. C. Since the full effect of the locking device will of course occur only when the connector is employed for the first time, the structure is not appropriate for example for a set screw, which often has to be readjusted.
Applying a resilient and deformable plastic to the external or internal thread in order to contribute to the moment of friction is also known. Since a locking device of this type will deform and remain deformed when first employed, however, absolute reliability can no longer be expected when the connector is activated and employed again.
Locking components of the type addressed herein can also be obtained for example by providing the supporting surface of the head of a headed bolt or screw with teeth to increase the moment of friction at that point. The teeth lock into the matching support surface of the work piece. Prerequisite, however, is that the teeth can work themselves into the work-piece material, which accordingly must not be harder that the stock that the bolt is made out of.
All of these known locking devices are subject to certain restrictions in relation to reusability, resistance to oil, solvents, and temperature, the material that the mating structure is made out of and its hardness, and persistence of action. A common drawback is the necessity for further processing or application of the locking device.
Providing the thread on the bolt and the thread on the nut with different pitches to distribute the load more evenly over the flights is on the other hand also known in relation to connectors of the nut-and-bolt type that lack locking devices to prevent unscrewing (Kaehler, "Means of distributing the load more uniformly over the supporting flights of a threaded connection" [in German], Konstruktion 12 [1952], 377-79). Uniform load on the separately connected thread flights can be obtained by making the pitch of the nut thread and the pitch of the bolt thread differ enough to be equal in the internal and external threads in relation to load per flight. When this ideal is approximated, the load on the initial supporting external thread is at least partly decreased. The difference between the pitch of the external thread and that of the internal thread is very small. A limiting condition for this distribution of forces is that the difference in pitches can never be greater than the flank clearance between the external and internal thread divided by the number of thread flights engaged. This ensures that the bolt can easily be mounted without grabbing.
Slightly different pitches between the external and internal threads have also been employed to increase the life of connectors of the nut-and-bolt type (Klein, "High-quality threaded connections: some principles of design and some new developments" [in German], Konstruktion [1959], 202-12 & 259-64). The improved distribution of forces and increased life depend on the known drawback that a conventional nut-and-bolt type connector, one with matching pitches that is, is essentially designed to transmit the forces in the vicinity of the first and of some of the following flights, whereas the other flights contribute nothing to the transmission of forces. Making the pitch of the external thread approximately 1 o/oo less steep than the pitch of the internal thread has the opposite effect and results in a more uniform distribution of forces over the individual flights along the depth of insertion. The consequence again is an effective extension of the life of the connection. There are of course practical problems in obtaining such slight differences in pitch to the requisite precision, and when the difference is too great the life of the connection will be shortened.